The present invention relates to a piezo-electric acceleration sensor for detecting acceleration of a diaphragm by means of a polymer piezo-electric element.
One typical example of the conventional piezoelectric acceleration sensor, using a polymer piezo-electric element, has a cantilever structure, in which a piezo-electric device P including the polymer piezo-electric element is supported by a supporting member S in a cantilever fashion as illustrated in FIG. 48(A). When a force F is applied to a weight M, mounted to the distal end of the piezo-electric device, strain is produced perpendicularly to the polarization axis. This piezo-electric acceleration sensor is disadvantageous in that when the polymer piezo-electric material is used as a diaphragm, resonance frequency becomes lower and the frequency of oscillation acceleration varies, thus varying the output of the sensor.
For improving the cantilever type piezo-electric acceleration sensor, an oscillation membrane of a film type sensor is proposed in Japanese Utility Model Application (kokai) No. 61-96340, in which a disc membrane piezo-electric element P and a diaphragm are sandwiched between ring-shaped fixing frames S in a laminated state. In this film-type sensor, overall oscillation characteristics may be improved by making the rigidity or deformation rigidity of the diaphragm sufficiently large relative to that of the polymer piezo-electric element P to thereby improve resonance frequency characteristics and to increase its output without deteriorating piezo-electricity of the piezo-electric element P. However, this type of sensor is insufficient in frequency characteristics and accuracy of acceleration detection in a low frequency range at low acceleration.
Japanese Utility Model Application (kokai) No. 61-99026 discloses a diaphragm membrane having a V-shaped flexible portion formed in a central portion thereof in a cantilever manner for improving thicknesswise oscillation detection characteristics. In this sensor, oscillation characteristics are considerably influenced by the V-shaped flexible portion, and hence it is liable to have the disadvantages of the cantilever-type sensor previously stated and it is likely that detection of oscillation frequency and amplitude in a wide range and detection of acceleration in a large temperature change environment become unstable.
Accordingly, it is an object of the present invention to provide a polymer piezo-electric acceleration sensor which provides stable output characteristics and frequency characteristics particularly at low acceleration and in a low frequency range.
It is another object of the present invention to provide a piezo-electric acceleration sensor which provides high sensitivity and high output.